1. Field of the Invention
The present invention relates to a stepping motor control system and a recording apparatus using the same.
2. Related Background Art
Conventionally, stepping motors are widely used as driving sources for industrial equipment in recent years due to their high rotational position alignment precision. In particular, stepping motors are popularly used as driving motors for so-called OA (office automation) equipment such as recording apparatuses.
As a typical control system of a stepping motor, a constant voltage driving system is known. This system requires a constant voltage circuit for preventing a variation in torque caused by a change in motor current corresponding to a change in voltage. As a result, (1) the size of the entire circuit becomes large and cost increases. Furthermore, since constant voltage driving is performed, (2) a vibration is generated in the motor, resulting in large rotation noise, and (3) electric power which is not used for driving is wastefully consumed, thus generating heat.
On the other hand, a constant current driving system is known as a driving system which is not influenced by a variation in voltage. In this constant current driving system, the value of a current flowing through a motor winding or coil is detected, and a switch element such as a transistor is pulse-width-modulation-driven so that the detected current value becomes a preset current value. The constant current driving system can solve the problem (1), but cannot solve the problems (2) and (3) due to constant current driving.
In order to solve the problems of the above-mentioned constant current driving system, the present applicant has proposed an open PWM control system in Japanese Patent Application No. 4-203863. In the open PWM control system, a driving current value set signal for setting a variable duty ratio by pulse-width modulation (PWM) is generated, and is supplied to each winding of a motor, so that a current value which corresponds to a vector component matching an arbitrary rotational position of the motor to some extent is supplied to each winding. More specifically, since it is theoretically ideal to supply a current value, which changes in a sine waveform pattern, to each winding, a current value with a staircase waveform which is approximates to the sine waveform as much as possible is supplied to each winding of the motor. Two-phase excitation driving in the open PWM control system will be explained below.
FIG. 21 shows a driving circuit of a stepping motor. The driving circuit shown in FIG. 21 comprises a micro-controller 201 for performing motor control, a pulse-width modulation unit (to be referred to as a PWM unit hereinafter) 202 which is incorporated in the micro-controller 201 and outputs pulse signals E and F whose frequencies and duty ratios can be set, an output port 203 which is incorporated in the micro-controller 201 and generates coded stepping motor control signals A, B, C, and D, a unipolar-coupled two-phase stepping motor 204, transistors 205 for exciting the stepping motor 204 in accordance with the control signals A, B, C, and D, current control transistors 206 for controlling currents flowing through the stepping motor 204 in accordance with the pulse signals E and F, fly-wheel diodes 207 for forming current paths when the current control transistors 206 are turned off, diodes 208 for preventing reverse currents due to induced voltages at the windings of the stepping motor 204, a programmable timer unit 209 incorporated in the micro-controller 201, and a ROM 210 which stores data such as the driving speed, the PWM duty ratio, and the like of the motor 204. The micro-controller 201 reads out such data from the ROM 210.
FIG. 22 shows the waveforms of the control signals for controlling the stepping motor 204. The micro-controller 201 generates the control signals A, B, C, and D for performing two-phase excitation driving of the stepping motor 204 via the output port 203. When these control signals are at the H level, the transistors 205 connected to these control signals are turned on, and the corresponding windings of the stepping motor 204 are excited. The change timing of each control signal, i.e., step time, is determined by the micro-controller 201 using the timer unit 209. By adjusting the step time, respective modes such as acceleration, deceleration, constant-speed operation, and the like are controlled.
The micro-controller 201 controls the PWM unit 202 to output the pulse waveforms E and F. These pulse waveforms are set to be pulse-output at a predetermined frequency (e.g., a frequency of 20 kHz or higher, which is higher than the audible range of a man) to have a predetermined duty at predetermined timings. The change timing of the duty ratio is also determined by the micro-controller 201 using the timer unit 209. When the pulse outputs are at the H level, the current control transistors 206 are turned on, and supply electric power to the motor 204. When the current control transistors 206 are turned off, electric power accumulated on in the motor 204 is discharged via the corresponding fly-wheel diodes 208. Upon repetition of these operations, the current to be supplied to the winding of the motor 204 can be controlled in accordance with the duty ratio of the pulse waveforms E and F.
FIG. 23 shows an example of PWM duty data stored in the ROM 210. Numerals 1 to 8 in the upper row are ROM addresses which are assigned for convenience, and numerical values in the lower row represent PWM duty ratios stored at respective addresses.
FIG. 24 shows the motor driving waveforms based on the PWM duty ratio data shown in FIG. 23. Note that signal waveforms E and F are not actual pulse waveforms, but express the duty ratios of pulses by their signal levels. Referring to FIG. 23, the micro-controller 201 changes the driving signals A and C, and sets the duty ratio of the PWM pulse signal E to be 40% in accordance with the numerical value, "40", stored at address 1 in the ROM 210. Thereafter, when the time 1/4 the step period has elapsed, the micro-controller 201 sets the duty ratio of the PWM pulse signal E to be 60% in accordance with the numerical value, "60", stored at address 2 in the ROM 210. Similarly, the micro-controller 201 sequentially reads out PWM data from the ROM 210, and sets the duty ratios. As for the PWM pulse signal F, values which are phase-shifted by 90.degree. from the signal E are set. Therefore, for example, when the signal E is set to have a value corresponding to data read out from address 1, the signal F is set to have a value corresponding to data read out from address 5. With the above-mentioned system, the current flowing through the motor can be controlled at a period 1/4 the step interval, and the same effect as that of a conventional driving system, known as a double 1-2 phase driving system can be obtained. More specifically, when the current waveform of the motor approximates a sine waveform, the motor can be operated with high efficiency and low vibration. Since the current flowing through the motor is influenced by the inductance of the motor winding and the counter-electromotive force, the PWM duty ratio is normally not proportional to the motor current. However, when duty ratio data in which these influences are corrected in advance are stored in the ROM 210, the current waveform can be controlled while approximating a sine waveform.
As described above, since the open PWM control system drives the motor based on a current value approximate to a sine wave, the problems (2) and (3) of the constant voltage driving system and the constant current driving system can be solved to some extent. However, these problems are not sufficiently solved, and the open PWM control system suffers the following problems.
(1) Since the open PWM control is an open system having no feedback loop, the pulse width of each PWM signal must be set by experimental trial and error so that the current flowing through the motor coil has a desired waveform.
(2) For example, a variation in value of a current flowing through each winding is large due to variations in the characteristics of transistors and diodes constituting the motor driving circuit. When the variation in current value is large, the variation in torque becomes large, and rotation nonuniformity occurs. As a result, constant-speed rotation cannot be attained. The current value varies due to a variation in source voltage, and the torque varies.
(3) In order to approximate the current waveform to an identical sine waveform as much as possible, a staircase waveform must be further smoothed. However, the number of times of switching of the level of the current value is eight per control signal, and a sufficiently smooth sine waveform cannot be obtained yet.
(4) A PWM memory table (a memory table of ON/OFF data) for the motor rotational speed is required, and a large volume of data must be stored in the memory (ROM).
On the other hand, a recording apparatus for performing recording on a recording sheet by scanning a carriage which mounts a recording head uses a stepping motor or a DC motor to scan the carriage.
In a recording apparatus, when a stepping motor is used in the above-mentioned open PWM control system, since high-precision, constant-speed rotation cannot be sufficiently obtained, printed (recorded) image nonuniformity may occur. In addition, since wasteful consumption power cannot be sufficiently eliminated, the service life of a battery cannot be prolonged in, e.g., a portable recording apparatus which receives electric power from a battery.
The DC motor is free from rotation nonuniformity at high speeds, and is suitable for constant-speed rotation. However, when a DC motor is used in a recording apparatus, a linear encoder for position detection, and a control circuit for processing a signal from the linear encoder and performing position control are required, resulting in a higher cost than an apparatus using a stepping motor.